Cancer is a term used to describe a group of malignancies that all share the common trait of developing when cells in a part of the body begin to grow out of control. Most cancers form as tumors, but can also manifest in the blood and circulate through other tissues where they grow. Cancer malignancies are most commonly treated with a combination of surgery, chemotherapy, and/or radiation therapy. The type of treatment used to treat a specific cancer depends upon several factors including the type of cancer malignancy and the stage during which it was diagnosed.
Gemcitabine is a commonly used cytotoxic agent that is used for the treatment of Pancreatic Cancer; Poplin et al J Clin Oncol, 27, 23, 3778-85, 2009 and Non-Small Cell Lung Cancer; Zinner, R G, et al., Int J Radiat Oncol Biol Phys, 73, 1, 119-27 2009; and Treat, J A, et al., Ann Oncol, 2009. Gemcitabine is also used as an adjuvant treatment in pancreatic cancer (Saif, M W, JOP, 10, 4, 373-7 2009; Li, J and M W Saif, JOP, 10, 4, 361-5 2009). Although it is widely used, this compound has been associated with debilitating side effects such as myelosupression, along with liver and kidney damage. By monitoring the levels of gemcitabine in the body and adjusting the dose these side effects can be better controlled and limited in patients.
Gemcitabine is the hydrochloride salt of the formula:

There is often high variable relationship between the dose of gemcitabine and the resulting serum drug concentration that affects therapeutic effect. This is especially prevalent in women and elderly patients. These groups display a lower clearance, resulting in higher plasma concentrations for any given dose. Gemcitabine (I) is metabolized in the body by cytidine deaminase (CDA) to its main pharmaceutically inactive metabolite: 2′,2°-difluoro-2′-deoxyuridine (dFdU) which has the formula:

There are at most only trace amounts of other gemcitabine metabolites in the blood, plasma or serum of patients. In preparing human biological samples such as blood and plasma samples for immunoassays it is necessary to use tetrahydrouridine (THU). This preservative acts to inhibit cytidine deaminase activity during the collection of patient samples to prevent further metabolism of gemcitabine to the inactive metabolite of the compound of formula II. The preservative tetrahydrouridine has the following formula:

The degree of intra- and inter-individual pharmacokinetic variability of gemcitabine varies greatly and is impacted by many factors, including:                Organ function        Genetic regulation        Disease state        Age        Time of sampling,        Mode of drug administration, and        Technique-related administration.        
As a result of this variability, equal doses of the same drug in different individuals can result in dramatically different clinical outcomes, as illustrated below (Hon, Y Y and W E Evans, Clin Chem, 44, 2, 388-400 1998.). The effectiveness of the same gemcitabine dosage varies significantly based upon individual drug metabolism and the ultimate serum drug concentration in the patient. Therapeutic drug management would provide the clinician with insight on patient variation in both oral and intravenous drug administrations. With therapeutic drug management, drug dosages could be individualized to the patient, and the chances of effectively treating the cancer without the unwanted side effects would be much higher (Nieto, Y, Curr Drug Metab, 2, 1, 53-66 2001).
In addition, therapeutic drug management of gemcitabine would serve as an excellent tool to ensure compliance in administering chemotherapy with the actual prescribed dosage and achievement of the effective serum concentration levels. It has been found that variability in serum concentration is not only due to physiological factors, but can also result from variation in administration technique (Caffo, O, S Fallani, E Marangon, S Nobili, M I Cassetta, V Murgia, F Sala, A Novelli, E Mini, M Zucchetti and E Galligioni, Cancer Chemother Pharmacol, 2010).
Routine therapeutic drug management of gemcitabine would require the availability of simple automated tests adaptable to general laboratory equipment. Tests that best fit these criteria are immunoassays such as a radioimmunoassay and an enzyme-linked immunosorbent assay. However the corresponding antibodies used in these immunoassays must demonstrate a broad cross-reactivity to gemcitabine, without any substantial activity to non-pharmaceutically active gemcitabine metabolites and the preservative of formula III. In order to be effective in monitoring drug levels of gemcitabine, the antibody should be most specific to the active compound, gemcitabine and display very low cross-reactivity to no cross-reactivity to the non-pharmaceutically active metabolite, 2′,2′-difluoro-2′-deoxyuridine (the compound of Formula II) and the preservative tetrahydrouridine (the compound of Formula III).